There's gold in them there hills

In a field in the heart of Kent, near a hedgerow bordering a quiet country road, two men waving strange gadgets around have just made a major discovery: if it starts raining when you go metal detecting, it's hard to get your hood up over your headphones.

"You'll find detecting much easier if you switch the machine on," says Justin Deeks, a metal detectorist (MD) with 25 years' experience. He bites into a cheese and coleslaw sandwich as the drizzle eases off. "Here, let me." The Fisher 1266-X, a good model for beginners, clicks into life and after just one sweep there's a beep. We grab the trowel, sift the soil and soon another discovery is made: that metal detecting is a bit like surfing the net. Finding stuff is a doddle. The problem is, 99% of it is crap. That beep was for an iron nail.

In the popular imagination, metal detecting is hardly regarded as riveting; a hobby, perhaps, for people who find trainspotting too full-on. This may be unfair. Since its birth in the 1970s, the story of metal detecting in Britain has all the excitement, conflict and skullduggery of an Indiana Jones movie, though sadly not the weather. It is a tale of midnight raids and fortunes made, legally and illegally. It features a bitter feud - fuelled in part by class snobbery and punctuated with arrests - that almost saw the pursuit outlawed. And it culminates in an incomparable harvest of gold, silver and bronze artefacts - not only dazzling in themselves, but priceless for what they can tell us about ancient Britain and beyond. All this from a machine that looks like a mop with a Stylophone on top.

In 2000, there were 289 treasure finds. A mere 13 were by archaeologists; the remaining 276 were by members of the public. This month the British Museum in London celebrates this debt with Treasure: Finding Our Past, the country's biggest archaeological exhibition for 20 years, and one that is 70% the fruit of MDs' patient labours in the ploughed fields of Britain. It is a way for the archaeological community to say thanks. And it is also, perhaps, a way for them to say sorry.

Relations between the two groups have not always been amicable. Imagine spending three years studying archaeology at university and then going off into the big wide world - only to have your thunder stolen by some amateur with a big stick. The combination of fear and snobbery set these two groups on a collision course.

"Don't worry," says Deeks, as a police car blares along the road bordering our field. "They wouldn't bother with the siren if they were coming for us." They might have done at one time, though. When the hobby boomed in the 1980s, with almost 30,000 people regularly pulling on their wellies and having a go, it started to get right up the noses of archaeologists. Uniting under the slogan "Stop stealing our past", they accused MDs of disturbing sites and of keeping finds for themselves (illegal if they contained enough precious metal to qualify as treasure). Boot MDs off your land, they implored farmers.

Suddenly Britain had a new villain: the nighthawk, a term given to any unscrupulous MD who searches known archaeological sites, usually after dark. They were a tiny minority, but things came to a brutal head at the Battle of Wanborough Temple. In 1983, two MDs found gold and silver Iron Age coins in a lane in Surrey and declared them to Guildford Museum. Unfortunately, at the coroner's treasure inquest, the location of the find was accidentally revealed - and the nighthawks swooped. Running rings round the police (although there was the odd arrest), this hardcore faction of ruthless MDs ransacked the site and made off into the night with the booty. It was archaeological armageddon. Many called for the hobby to be banned.

Had their wish been granted, we might never have had the Winchester Gold, discovered in 2000 by retired florist Kevan Halls. Showcased in Finding Our Past, Halls' haul of gold brooches and necklaces - which earned him and the landowner £350,000 - changed our view of Iron Age Britain for ever. So exquisitely crafted were the flexible necklaces, probably dating from the first century BC, it is believed their maker was trained in the Mediterranean, or that they were acquired from a Roman patron. Either explanation points to unexpected and unprecedented links between Britain and the classical world.

Nor would we have had the Ringlemere Gold Cup (also in the exhibition), which was found in Kent by retired electrician Cliff Bradshaw in 2001. Estimated at the time to be worth about £250,000, the superbly crafted piece - bashed, sadly, probably by a plough - came up out of the soil like a ray of sunshine, as dazzling as the day it was buried 3,600 years ago, since gold in its pure form never corrodes. All it needed was a shake to get the dirt off.

Nor, indeed, would we have the find that earned Eric Lawes, a retired gardener, £1.75m in 1992. Dating from AD410-30, the staggering Hoxne Hoard is the largest cache of Roman gold and silver ever discovered on British soil, comprising rings, chains, bracelets, tableware and 15,000 coins. Much of it is on show in Finding Our Past.

These discoveries were made possible because one good thing came out of the Battle of Wanborough Temple: MDs and archaeologists realised they had to work together. The result was a change to the ancient treasure trove law, the oldest in the British isles and hopelessly outdated. Previously, any objects of gold and silver for which a legal owner could not be traced were claimed by the Crown, with no reward required for finders. Sometimes - as in the case of Londoner Peter White, who in 1400 found 35 shillings - they were treated as criminals and banged up in jail. Consequently, many finds went undeclared and many valuable artefacts wound up in the melting pot. In 1810, writes curator Richard Hobbs in his book accompanying Finding Our Past, a Cornish girl discovered an ancient gold chain - and used it as a whip to drive her cattle before her brother sold it to a jeweller for £3. Among other things, the 1996 Treasure Act decreed that, if an object is wanted for a museum, a sum based on its auction value should be split between finder and landowner.

But metal detecting isn't just about treasure. Deeks, 38 years old and an office worker with the fire brigade, occasionally helps out the RSPCA: "If they fire a tranquilliser dart at an animal, say an escaped deer, and it misses, we find it for them. It's not the sort of thing you want lying around." And he once went detecting in a pond that had been drained because it had silted up. "They were about to dredge it," he says cheerily. "I found a live hand grenade in the mud. It was like a baked bean tin on a stick. The digger would have hit it and blown up." Who says metal detecting is boring?

· Treasure: Finding Our Past is at the British Museum, London WC1, from Friday to March 14. Details: 020-7323 8000. Then tours to Cardiff, Manchester, Newcastle and Norwich. The Federation of Independent Detectorists: www.newbury.net/fid